1. Field of the Invention
The present invention relates to a thin film transistor liquid crystal display, and more particularly a photo mask for fabricating a thin film transistor liquid crystal display using 4-mask process.
2. Description of the Related Art
Thin Film Transistor Liquid Crystal Display (hereinafter referred as TFT-LCD) has advantages of light weight, thin thickness and low power consumption. Therefore, it has been substituted for Cathode-ray tube (CRT) in a terminal of information system and video unit, etc. and recently, it is widely used in a notebook PC and a computer monitor market.
This TFT-LCD comprises a TFT array substrate which has a structure that TFT is disposed on each pixel arranged in a matrix shape, a color filter substrate which has a structure that red, green and blue color filters are arranged corresponding to each pixel, and a liquid crystal layer interposed between the substrates.
In fabrication of the TFT-LCD, it is very important to reduce the number of fabrication processes, particularly, the number of TFT array substrate fabrication processes. This is because reduction of fabrication processes lowers production cost, thereby increasing the TFT-LCD supply with a low price.
In order to reduce fabrication processes, it is necessary to reduce the number of photolithography processes and it is realized by reducing the number of photo mask used in the processes. Recently, photo masks of 5 to 7 sheets are used for fabricating TFT-LCD, and photo masks of 4 sheets may be used in some cases.
FIGS. 1A to 1E are cross-sectional views for showing a conventional method of fabricating a TFT array substrate using photo masks of 4 sheets. TFT formation parts are illustrated in the drawings.
Referring to FIG. 1A, a gate metal layer is deposited on a glass substrate (1), and then a gate line (not shown) including a gate electrode (2) is formed by patterning with a mask process using a first photo mask. A gate insulating layer (3), an axe2x80x94Si layer (4), a n+ axe2x80x94Si layer (5), a source/drain metal layer (6) and a sensitive layer (7) are sequentially formed on the glass substrate including the gate electrode.
Referring to FIG. 1B, the sensitive layer (7) is exposed using a second photo mask and the exposed sensitive layer is then developed, thereby forming a sensitive layer pattern (7a) to cover a channel unit and source/drain formation regions. The sensitive layer pattern (7a) is formed by half tone exposure and the center thereof, that is, a part to cover a TFT channel formation region, is thinner than parts to cover source/drain formation regions.
According to the half tone exposure, each region is exposed to different exposure degree, so that a photoresist pattern has uneven thickness. In this process, the exposure degree may be controlled by designing a photo mask.
FIGS. 2A and 2B are respectively a cross sectional view and a plane view generally illustrating a photo mask for the half tone exposure process. As shown in FIGS. 2A and 2B, a photo mask (20) for the halftone exposure comprises a light transmission substrate (11) and a shielding pattern (12). In addition to a light transmission region (A) and a light shielding region (B), a semi-permeable region (C) is included, which semi-permeable region transmits light at lower degree than that transmitted by the light transmittance region (A).
The resolution of a stepper as an exposure device is 3 xcexcm. Therefore, when fine patterns with resolution lower than that of the exposure device are formed on the transparent substrate (11), the degree of exposure is lowered in the formation region of the fine patterns, thereby the sensitive layer pattern corresponding to this region has a lesser thickness when compared to other regions.
Referring to FIG. 1C, source/drain metal layers are etched using the sensitive layer pattern (7a) as an etching mask to form a data line.
Referring to FIG. 1D, n+ a-Si layer (5) and a-Si layer (4) are etched using the sensitive layer pattern (7a) as an etching mask to define an active region, and subsequently, source/drain (6a, 6b) are formed by etching source/drain metal layer disposed on the channel region. That is, while the n+ a-Si layer and the a-Si layer are etched, center of the sensitive layer pattern (7a) is etched together with the layers due to relatively lesser thickness and the source/drain metal layers of the exposed channel region are also etched to form the source/drain (6a, 6b).
Referring to FIG. 1E, a n+ axe2x80x94Si layer on the exposed channel region is etched and then, the sensitive layer pattern is removed, thereby completing a TFT (10).
Although it is not shown in drawings, a protective layer is formed using a third photo mask and a pixel electrode is formed using a fourth photo mask. A TFT array substrate is completed through well-known following processes including formation processes of the protective layer and the pixel electrode.
However, a conventional method of fabricating a TFT array substrate has following problems.
As shown in FIG. 3, in half tone exposure, both edges of a channel unit (4a) are flexed or diffracted by interference of light since there is no clear division between a light transmittance region and a light shielding region at the edge of channel region. When edges of the channel unit (4a) are diffracted, a path of On-current is diffracted at the diffracted edges of channel unit (4a), thereby causing deterioration in the quality of the TFT-LCD screen.
Therefore, an object of the present invention is to provide a photo mask for fabricating TFT-LCD which can prevent diffraction at both edges of the channel region in halftone exposure.
In order to achieve the above object, a photo mask for fabricating TFT-LCD according to an embodiment of the present invention comprises a transparent substrate and a shielding pattern formed thereon, wherein the shielding pattern includes: a pair of first shielding patterns each having the rectangular shape disposed with separation therebetween to cover a source and a drain formation regions; a pair of second shielding patterns each having the longitudinal shape of the bar disposed between the first shielding patterns; and third shielding patterns each having the longitudinal shape of the bar being tansversely disposed adjacent upper and lower portions of the first and second shielding patterns to divide a light transmittance region and light shielding region at the edges of a channel region.